This invention relates to a so-called single station type fire alarm system which comprises an electric source, a fire detecting section, a warning unit such as a buzzer, etc. in one housing to detect a fire and to produce a warning signal. This invention also relates to a multivibrator used in the detection circuitry.
A fire alarm system of this type is provided with detectors adapted to detect smoke, heat and flame, a warning unit such as a buzzer or a bell adapted to produce a warning signal in response to the outputs of the detectors. An electric source such as a dry battery for energizing these components is located in one housing. Such a fire alarm system is installed, as a simple warning signal producing device for home use, at crucial locations in a house such as landings or stairways. That is, the fire alarm systems are provided in rooms separately or independently from each other without having a central control device.
Accordingly, even if one fire alarm detects a fire in the respective location of a room to produce a warning signal, the warning signal cannot be transmitted to another fire alarm at another location. Since the rooms in recent houses are well insulated in terms of soundproofing and airtightness, it may be difficult for a person in one room to hear the warning signal produced in another room. Hence, at a crucial moment, such as a spreading fire, only the persons in the room where the alarm sounds can be alerted to the occurrence of the fire at the earliest moment. This may result in delay in extinguishing the fire or in evacuation from the building.
Originally, the single station type fire alarm system of this type was used as a simple warning signal producing device as described above, and therefore it was not intended to have a mutual warning signal control function. The increase in control functions leads to an increase in power requirements. Hence, a problem generally exists concerning acceptable battery lifetime without requiring large and heavy power sources. Hence, the use of circuitry having reduced power is a subject of continuing research.
This invention therefore also relates to a multivibrator employing C-MOS IC inverters, and is intended to reduce the power consumption of the multivibrator. A C-MOS IC has a variety of merits in that it is low in power consumption, may be completely operated by batteries, and is not affected by external noise. Therefore, C-MOS IC circuitry has been extensively utilized in single station type fire alarm systems using a primary battery as its electric source.
For instance, in a single station type fire alarm system incorporating an ionization type smoke detector, a DC--DC converter using a multivibrator with C-MOS inverters is employed to obtain a high voltage applied to the ionization chambers of the detector. Since the multivibrator is a load on the battery, it is desirable to minimize the power consumption of the multivibrator. Most of the multivibrators of this type have an arrangement as shown in FIG. 1. More specifically, in this arrangement, the periodic variation of a potential Va at the circuit point a due to a differentiation current flowing in a resistor 3 and a capacitor 4 is obtained as a pulse at the circuit points b or d by means of two C-MOS inverters 1 and 2.
The operation of this conventional multivibrator shown in FIG. 1 will now be described. As is apparent from a waveform diagram shown in FIG. 2, the potential Va at the circuit point a periodically varies at the time instants t.sub.0, t.sub.1, t.sub.2 and so on with a time constant defined by the values of resistor 3 and capacitor 4. At the time instant t.sub.0, the output of the inverter 1 is changed from a high potential level "H" (hereinafter referred to merely as an "H" when applicable) to a low potential level "L" (hereinafter referred to merely as an "L" when applicable), while the output of the inverter 2 is changed from "L" to "H". Therefore, capacitor 4 has been biased in such a manner that the circuit point d is negative and the circuit point a is positive. But, at the time instant this biasing state is inverted, and the potential Va is increased to about (V.sub.DD +V.sub.TH).
During the period of from t.sub.0 to t.sub.1, a differentiation current flows from the P channel MOS of inverter 2 through the capacitor 4 and resistor 3 to the N channel MOS of inverter 1. The potential is gradually decreased from "H". At the time instant t.sub.1, the potential Va, gradually decreased, reaches the circuit threshold voltage V.sub.T of inverter 1. As a result, the states of inverters 1 and 2 are both inverted, and the potential at the circuit point b is changed from "L" to "H" while the potential Vd at the circuit point d. Accordingly, capacitor 4 is reversely biased again and the potential Va is instantaneously decreased to about -V.sub.TH.
During the period of from t.sub.1 to t.sub.2, a differentiation current flows from the P channel MOS of inverter 1 through resistor 3 and capacitor 4, in the stated order, to the N channel MOS of inverter 2. The potential Va is gradually increased. At the time instant, t.sub.2, the potential Va reaches the circuit threshold voltage V.sub.T, and the same state as that in the case of the time instant t.sub.0 is provided.
As the above-described operation is repeatedly carried out, the output provided, for instance, at the circuit point d is an oscillation output having a constant period. In FIG. 2, a curve I indicates the variation with time of the current consumption of the circuit. The variation tends to increase when the potential Va reaches approximately the circuit threshold voltage of inverter 1. Accordingly, especially in the case where the oscillation period of the multivibrator is set to a short value, the current consumption cannot be disregarded if the variation occurs slowly when the potential Va passes the above described threshold voltage.
Within the prior art a number of patents are known generally relating to this technology. Typical pertinent ones are U.S. Pat. Nos. 4,004,288; 4,017,852; and 4,030,086. The '288 patent discloses a battery operated detection unit for independent monitoring of smoke. A smoke detector is used to produce an output to latch circuitry and a logic gate to produce an audiable alarm. An oscillator is also coupled to the logic gate such that in the absence of an alarm signal, a light is energized at a rate dependent on oscillator frequency to indicate that the system is in an operable state. A battery monitoring circuit shown in FIG. 5 of the patent is employed to provide the audible alarm if the battery voltage drops below a predetermined level. Such a system, while workable suffers from several practical drawbacks. It is not capable of being adapted into a network and the user has no way of initially noting the difference between an alarm condition or one of low battery. Also, the system requires relatively large amounts of power for operation.
The '852 patent relates to a smoke detector having interconnect lines to couple a number of different units as shown in FIG. 2 therein. The device uses a network of silicon controlled rectifiers (SCR) and a programmable unijunction transistor (PUT) in the detector and supervision circuits. The device is not self powered since it uses line connections. Accordingly, as shown it must use a transformer (element 12 in FIG. 1) to handle the input line voltage. Such a device cannot therefore be placed at random or selective locations but must be near a power line. This leads to unsightly wires and installation problems. The convenience of installing detectors in locations specific for their function such as in stair wells is lost unless a power line is present.
Finally, the reference Salem ('086) relates to a battery operated smoke detection device with a monitoring circuit for sensing low battery condition. The patent relates specifically to a device that will provide a permanent visual alarm and notice that the battery should be replaced. This is in the form of an indicator 96' shown in FIG. 2 of the patent held in a closed position by a tab 125 biased with a spring 112 and an energizable coil 92 coupled to a latch 120. When the low battery circuit sense a condition below a predetermined threshold, the latch 120 releases the tab 125 and the indicator 96' becomes visable. Thereafter and until the battery is depleted a horn 50 will sound as a result of discharge of capacitor 82 to provide an audiable warning signal. Therefore, even if the battery is completely exhausted the visual warning will remain. This patent while showing a reliable and efficient low battery sensor is not capable of network operations.